1. Field of the Invention
The present invention relates to laminated balun transformers, and more particularly, to a laminated balun transformer for use as a balanced-to-unbalanced signal converter, a phase transformer, or other suitable device for an IC for a radio communication apparatus.
2. Description of the Related Art
A balun transformer is used to convert, for example, a balanced signal from a balanced transmission line into an unbalanced signal for an unbalanced transmission line or to convert an unbalanced signal from an unbalanced transmission line into a balanced signal for a balanced transmission line. The term xe2x80x9cbalunxe2x80x9d is an abbreviation of xe2x80x9cBALanced-UNbalancedxe2x80x9d.
A balanced transmission line has a pair of signal lines, through which a signal (a balanced signal) propagates as a potential difference therebetween. In a balanced transmission line, the two signal lines are equally affected by external noise, thereby canceling out the external noise. Thus, the balanced transmission line has an advantage in that it is less susceptible to the influence of external noise. In addition, since the internal circuit of an analog IC includes a differential amplifier, the input/output terminals for a signal for the analog IC are, in many cases, of a balanced type that outputs or inputs a signal as a potential difference between the two terminals.
In contrast, in an unbalanced transmission line, a signal (unbalanced signal) propagates as a potential of a single signal line relative to a ground potential (zero potential). Examples of an unbalanced transmission line include a coaxial line and a microstrip line disposed on a substrate.
Conventionally, a laminated balun transformer 1 shown in FIG. 7 has been proposed as a balanced-to-unbalanced converter for a transmission line in a radio-frequency circuit.
The balun transformer 1 includes dielectric sheets 2a to 2h. On the surface of the dielectric sheet 2b, a lead electrode 3 is provided. On the dielectric sheets 2c, 2d, 2f, and 2g, xc2xc wavelength strip lines 4, 5, 8, and 9 are provided, respectively. On the surfaces of the dielectric sheets 2a, 2e, and 2h, shield electrodes 12, 13, and 14 are provided, respectively.
The strip lines 4 and 9 are electrically connected in series through a relay terminal N, which is provided on an outer surface, to constitute an unbalanced transmission line. The strip lines 5 and 8, on the other hand, each constitute a balanced transmission line.
The strip line 5 is arranged so as to oppose the strip line 4 with the sheet 2c being interposed therebetween. Thus, the strip lines 4 and 5 are electromagnetically coupled to constitute a coupler. The strip line 9 is arranged so as to oppose the strip line 8 with the sheet 2f interposed therebetween. Thus, the strip lines 8 and 9 are electromagnetically coupled to constitute a coupler. The strip lines 5 and 9 are connected in series through a via hole 18 that is formed in the sheets 2d and 2e.
Meanwhile, in some cases, a balun transformer 1 for use in an mobile communication apparatus, such as a portable telephone, or a wireless LAN are required to amplify a balanced signal by applying a bias to only the balanced transmission lines.
In the balun transformer 1 described above, however, one end of the strip line 5 and one end of the strip line 8, which constitute the balanced transmission lines, are electrically connected to a shield terminal G via a shield electrode 13. Thus, it is impossible to apply a bias voltage to only the balanced transmission lines. In other words, the balun transformer 1 having such a configuration has a problem in that a balanced signal cannot be amplified.
In order to overcome the problems described above, preferred embodiments of the present invention provide a laminated balun transformer that allows a bias voltage to be applied to only transmission lines to amplify a balanced signal.
According to a preferred embodiment of the present invention, a laminated balun transformer includes a stack in which, at least, a pair of balanced transmission lines, an unbalanced transmission line that is electromagnetically coupled with the pair of balanced transmission lines, a shield electrode that opposes at least one of the balanced transmission lines and the unbalanced transmission line, and a plurality of dielectric layers are stacked, a ground terminal that is provided on a surface of the stack and that is electrically connected to the balanced transmission lines, and a shield terminal that is provided on a surface of the stack and that is electrically connected to the shield electrode.
One of the unique features of the laminated balun transformer is that the ground terminal and the shield terminal are electrically independent of each other.
With this arrangement, since the ground terminal that is electrically independent of the shield terminal is provided, for example, a bias voltage can be applied to only the ground terminal.
In addition, another preferred embodiment of the present invention provides a laminated balun transformer that includes a pair of balanced transmission lines, an unbalanced transmission line that is electromagnetically coupled with the pair of balanced transmission lines, an unbalanced signal terminal that is electrically connected to one end of the unbalanced transmission line, two balanced signal terminals, each terminal being electrically connected to one end of one corresponding balanced transmission line, a common bias terminal that is electrically connected to the other ends of the balanced transmission lines, and a bias capacitor that is electrically connected between the common bias terminal and ground.
In addition, another preferred embodiment provides a laminated balun transformer that includes a stack in which, at least, a pair of balanced transmission lines, an unbalanced transmission line that is electromagnetically coupled with the pair of balanced transmission lines, a shield electrode that opposes at least one of the balanced transmission lines and the unbalanced transmission line, a pair of bias-capacitor patterns, and a plurality of dielectric layers are stacked, a common bias terminal that is provided on a surface of the stack and that is electrically connected to the balanced transmission lines and one capacitor pattern of the bias-capacitor patterns; and a shield terminal that is provided on a surface of the stack and that is electrically connected to the shield electrode and the other capacitor pattern.
One of the unique features of this laminated balun transformer is that the common bias terminal and the shield terminal are electrically independent of each other.
With the unique arrangements described above, when a bias electrode is applied to the common bias terminal, the bias electrode is applied to only the balanced transmission lines via the bias capacitor, thus allowing for stable amplification of a balanced signal.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.